The present invention relates to a method for performing a cell update procedure for a station in a cellular communications system. The invention relates further to an arrangement in a cellular communications system and an apparatus for use in the cellular communications system for implementing cell update procedures.
A cellular radio network system, such as the proposed CDMA (Code Division Multiple Access) or UMTS (Universal Mobile Telecommunications System) or various GSM (Global System for Mobile communications) based systems (such as GPRS: General Packet Radio Service), consists of cells. In most cases the cell can be defined as a certain geographically limited area covered by one or several base stations (BS) serving mobile stations (MS) via an air interface and connected to a base station subsystem (BSS). Several cells cover a larger area, and form the coverage area of a cellular radio network. E.g. in the proposed CDMA standard each cell is controlled by a radio network controller (RNC). The mobile station (MS) or user equipment (UE) within one of the cells of the system is controlled by a serving radio network controller (SRNC) which is connected further to a gateway apparatus, such as a serving GPRS support node (SGSN), linking the cell to the other parts of the communication system. In addition to the SRNC, the MS may also be connected to a drift RNC (DRNC). Where the MS is in soft handoff and thus in communication with two base stations, those base stations may be connected to different RNCs. One RNC is the serving RNC (SRNC) and one is the drift RNC (DRNC). The DRNC is located in the communication path between the BS, which it controls, and the SRNC. The control of the connection to the other parts of the communications system will be accomplished by the SRNC.
In a xe2x80x98cell updatexe2x80x99 mode or similar situation, i.e. when the MS in RACH/FACH (Random Access Channel/Forward Access Channel) mode enters a new cell (i.e. moves from the coverage area of one cell to a next cell or is switched on), the MS sends a message containing a MS (or UE) identifier (ID). This MS ID is valid in all networks conforming the standard in question. In the case of the proposed CDMA standard, the MS ID is valid in all UTRANs (Universal Mobile Telecommunications System Terrestrial Radio Access Network). This MS ID comprises the SRNC ID identifying the particular SRNC and the sRNTI identifying the MS within the SRNC. This sRNTI is the identifier used by the SRNC within the SRNC. This MS ID is sometimes referred to as routing information.
There are proposals that this MS routing information message could be in a form of a xe2x80x98Cell Update Request Messagexe2x80x99 where the MS requests that the cell be updated. Alternatively this message could also be part of a message which also contains user data.
The RNC controlling the new cell (i.e. the target RNC) may be different from the one that controlled the previous cell. The previous cell may be controlled by a DRNC or a SRNC. The target RNC may be different from the SRNC. The following concerns cases where the target RNC and the SRNC are not the same.
This MS routing message may be sent by the MS by using CCCH (Common Control Channel) logical channel or DCCH (Dedicated Control Channel) logical channel. The CCCH uses only MAC-c services. Located in the target RNC, MAC-c (Medium Access Control-c) is a controller which controls the RACH/FACH channels. The DCCH messages use the services provided by MAC-c and RLC (Radio Link Controller) which are also located in the SRNC.
Reference is now made to FIG. 1 which shows the L2 (Layer 2 of the protocol stack of ISO OSI model: link layer) model functional split between the SRNC 100 and the DRNC 102. More particularly, FIG. 1 shows the MAC units 104 and 106 of the SNRC 100 and the MAC units 108 of the DRNC 102. The MAC (Medium Access Control) function is split across Iur interface 110 between the DRNC 102 and the SRNC 100. The SRNC 100 and DRNC 102 each have a Radio Resource Controller RRC 114 and 116, respectively. The RNSAP (Radio Network Subsystem Application Part; in the control plane) protocol is used for pair to pair L3 (Layer 3) signalling between the RRCs 114 and 116 of the SRNC and the DRNC. In the SRNC 100, there is a PCCH (Paging Control Channel) between the RRC (Radio Resource Control) 114 and a first MAC unit 114. The first MAC unit outputs a PCH (Paging Channel) to a base station. A CCCH (Common Control Channel) is provided between the RRC 114 and the second MAC unit 106. The second MAC unit 106 outputs the FACH (Forward Access Channel) and receives the respective RACH (Random Access Channel). In the DRNC 102 DCCH (Dedicated Control Channel) is provided between the RRC 116 and the MAC units 108. Even though not shown, a RLC (Radio Link Control) is located on the DCCH between the RRC 116 and the MAC units 108. The Frame Protocol (user plane) 112 is used to carry MAC-d PDU (Protocol Data Unit) between the drift and serving RNCs.
After the SRNC has been notified about the access of the MS to a new cell, the SRNC makes a decision whether to perform a relocation (i.e. a new SRNC is required) before sending a downlink ACK (acknowledgement) to the mobile station MS, or whether there is no need for relocation proceedings. If relocation is required, the serving RNC will change. If there is no need for relocation proceedings and the SRNC and the RNC controlling the new cell are different, the RACH/FACH data streams from and to the MS will then be conveyed on the Iur interface between the SRNC and the new RNC via Iur Frame Protocol.
In the earlier proposals it has been suggested that the initial access message be transmitted by the MS. The message, which is hereby referred to as the xe2x80x98Cell Updatexe2x80x99 message, may be any message containing the required routing information in order to be able reach the SRNC. The routing information consists of the SRNC ID and the sRNTI as discussed above.
One possible solution for implementing the above could use the MAC model such that the Cell Update message is sent on the CCCH from the MAC of the target RNC to the RRC of the target RNC and carried over RNSAP to the SRNC. Even though this would be easy to implement, the MS, however, cannot send to the new target RNC an initial access message which requires MAC and RLC services (Radio Link Control services). Those MAC and RLC services can include ciphering, RLC retransmission (especially if the message contains user data) or an authentication and/or integrity check. Authentication is security feature used for ensuring the identity of the transmitting party, and integrity check can be used for verifying that the contents of the transmission correspond the original message. Both of these could be a part of RLC or RRC (Radio Resource Control).
According to another proposal, the Cell Update message could be sent over the DCCH between the MAC and the RRC of the target RNC, and the MAC PDU of the target RNC could then be carried in the RNSAP message from the RRC of the target RNC to the RRC of the SRNC. However, carrying the MAC PDU in L3 messages is not compliant with the current protocol stack. From FIG. 1 it can be seen that MAC-d PDUs are carried via Frame Protocol (FP), not via RNSAP. This solution could cause some problems in view of the current RNC implementation proposals.
According to another proposal, the Cell Update message could be sent over DCCH between the MAC and the RRC of the target RNC. A CRNC (Controlling Radio Network Controller: the RNC including the MAC-c for the cell) could start the setup of the CCH FP and the MAC-d PDU is conveyed from the MAC of the SRNC using the Frame Protocol (FP). The problem with this solution is that the CCH support over the Iur would require a new protocol (user plane on the Iur interface) that is not otherwise needed if the SRNC decides to perform the relocation (after which the user plane will become useless). The user plane is not needed also if the message is e.g. a URA (UTRAN Registration Area: a group of cells) update.
It is noted herein that the above proposals do not necessarily constitute the state of the art.
It is an aim of the embodiments of the invention to address one or several of these problems.
According to one aspect the present invention provides a method of performing a cell update procedure for a station in a cellular communications system, the cellular communication system comprising a plurality of cells, and a first and a second network controllers, said station initially being on a first cell controlled by the first controller and then in a second cell controlled by the second controller, the method comprising the steps of:
sending, when the station is in the second cell of the system, cell update information and other information from the station to the second controller;
storing the received other information at the second controller; and
sending from the second controller to the first controller a message indicating that cell update information has been received at the second controller from the station.
According to a further aspect, the present invention provides an arrangement in a cellular communications system, the cellular communications system comprising a plurality of cells, a first controller controlling a first cell of the plurality of cells, a second controller controlling a second cell of the plurality of cells, and a station capable of communicating with said controllers, comprising:
means for sending, when the station changes from communicating with the first controller to communicate with the second controller, cell update information and other information from the station to the second controller;
storage means at the second controller for storing the received other information; and
means for creating and sending from the second controller to the first controller a message indicating that cell update information has been received at the second controller from the mobile station without sending said other information to the first controller.
According to a still further aspect, the invention provides an apparatus for use in a cellular communications system, the cellular communications system comprising a plurality of cells, a first controller controlling a first cell of the plurality of cells, a second controller controlling a second cell of the plurality of cells, and a station capable of communicating with said controllers, wherein the cellular communications system is such that when the station changes from communicating with the first controller to communicate with the second controller cell update information and other information is sent from the station to the second controller, said apparatus comprising:
storage means at the second controller for storing the other information upon receipt at the second controller; and
means for creating and sending from the second controller to the first controller a message indicating that cell update information has been received at the second controller from the mobile station.
By means of the proposed solution it may be possible to perform the cell update from one cell to a new cell controlled by another controller (for example, a drift RNC) in a cellular communication system such that the CCH user plane does not need to be a mandatory part of the interface between the new and serving RNCs. The MS may use data units (e.g. DCCH or DTCH) for the access to the new cell. This enables, for example, the MS to send user data or use authentication and/or integrity checks which could not be possible in the above discussed earlier proposals. By preventing establishment of unnecessary user plane connections and using a data packet unit instead of conventional signaling it may also be possible to reduce the amount of signaling load caused by roaming user equipment or mobile stations.
Embodiments of the present invention may avoid use of a mandatory user plane in the interface between a new RNC and a serving RNC. More generally, embodiments of the present invention may avoid the establishment of unnecessary user planes between radio network controllers of the cellular radio communications system. In addition, embodiments of the invention may allow the amount of signaling load caused by roaming mobile stations to be reduced, especially between a drift RNC and a serving RNC.